Process for fabricating porous beryllium billets

ABSTRACT

Porous beryllium billets are prepared by cold isostactic pressing of a mold containing beryllium powder, and sintering under a vacuum the resulting beryllium billet. The billets so produced are particularly useful as a matrix material for active transpiration systems.

ilnited S taies Patent 1 Rosenwasser represented by the Secretary of the Air Force, Washington, DC.

22 Filed: Mar. 15, 1973 21 Appl. No.: 341,634 7 [52] US. Cl 75/221, 75/200, 75/214, 75/225, 75/222, 148/126, 29/182 [51] Int. Cl 1322f 3/16 [58] Field of Search..... 75/214, 221, 200, 225, 222; 148/126 [56] References Cited UNITED STATES PATENTS 2,794,241 6/1957 Dodds et a1 75/221 [111 3,793,014 [4 1 Feb. 19,1974

2,818,339 12/1957 Dodds ..75/225 OTHER PUBLICATIONS Hausner et al., The Powder Metallurgy of Beryllium ASM 1950 Preprint No. 38

Primary Examiner-Carl D. Quarforth Assistant ExaminerB. Hunt [5 7 ABSTRACT Porous beryllium billets are prepared by cold isostactic pressing of a mold containing beryllium powder, and sintering under a vacuum the resulting beryllium billet. The billets so produced are particularly useful as a matrix material for active transpiration systems.

4 Claims No Drawings FIELD OF THE INVENTION This invention relatesto a process for fabricating porous beryllium billets. In one aspect it relates to beryllium billets having a controlled permeability.

BACKGROUND OF THE INVENTION ties. As disclosed in the literature, porous beryllium can be produced by uniaxial cold pressing and sintering, uniaxial hot pressing, pressureless sintering and hot isotactic pressing. The uniaxial cold pressing and sintering method has been used for the consolidation of permeable matrices. However, inhomogeneities in green density arising from particle-to-particle and particle-to-die wall friction. leads to non-uniform permeability in the sintered material. The uniaxial hot pressing method was developed to produce consolidated berylliurn of maximum density (minimum porosity). However, the process is not adaptable to the production of uniform, controlled permeability beryllium because heterogeneous plastic deformation is produced by die wall friction. Pressureless sintering of beryllium powdersafter vibration to about 50 percent of theoretical density has been studied in detail, and several characteristics of the process, pertinent to its potential for producing controlled permeability beryllium, have been established. The final density of pressureless sintered compacts is dependent on small variations in impurity content. Furthermore, since this process is highly sensitive to temperature and the actual densification required is so great, it is extremely difficult to obtain reproducible, fine-grained, uniform permeability billets. Some controlled permeability porous beryl-' lium has been produced by the hot isostactic pressing of closely fractionated spherical powders. Although some uniformly permeable material was produced, nonuniform particle deformation resulting in heterogeneous pore structure was noted.

It is an object of this invention to provide a process for preparing porous beryllium billets possessing uniform, controlled permeability.

Another object of the invention is to provide a process for preparing porous beryllium billets having good structural properties. I

A further object of the invention is to provide porous beryllium billets having permeabilities in the range of l l0- to 1X10 in Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.

SUMMARY OF THE INVENTION Broadly speaking, the present invention resides in a process which comprises the steps of subjecting a mold filled with beryllium powder of a certain particle size to cold, e.g., room temperature, isostactic pressing; and sintering under a vacuum the resulting billet (green compact) at a temperature and for a period of time suitable for producing a porous beryllium billet having a desired permeability. This particular procedure, which can be conveniently referred to as the sin- 2 gle-pressed process, is suitable for preparing billets having permeabilities of 1X10 to 1X10 in In a preferred embodiment of the invention, particularly where it is desired to obtain billets having low permeabilities, e.g., lower than 1X10 in a procedure that can be termed a double-pressed process is followed. According to this process, the beryllium billet (green compact) recovered from a first cold isostactic pressing is annealed after which the billet is subjected to a second cold isostactic pressing. Thereafter, the billet is sintered under a vacuum, thereby providing a billet having a permeability lower than 10*" in. It has been found that this process promotes stress relief in the green compact, allowing further deformation on repressing that results in higher densities prior to sintering. Furthermore, the process improves uniformity at low permeabilities by minimizing the sintering temperature.

The beryllium powder used in preparing the billets has a particle size ranging between 74 and 10 microns (74+l0;1.). While other blends of beryllium power, e.g., 74+44p. and -44+10,u. powders can be utilized, for optimum and reproducible results it is important that the 74+l Op. powder be employed. Thus, all of the desired permeabilities can be produced with the -74+l0,u. powder by either the single or the double pressed process. However, as mentioned hereinabove, to obtain permeabilities lower than 1X10" in the double-pressed process must be used. Thus, the double-pressed process can be employed to fabricate billets having permeabilities ranging from 1X10 in to lXlO. It is noted that by setting the lower particle size limit at 10p. submicron particles are effectively eliminated during the powder classification process. The absence of submicron fines facilitates powder handling and increases the uniformity of the final product. Also, to obtain reproducible results, powder size distribution from lot to lot must be controlled. A -+l0pt powder blend containing between 10 and 16 percent by frequency of particles less than 10p. has been found to provide improved reproducibility.

In the single-pressed process, the isostactic pressing step is conducted at a pressure in the range of 40 to 60 ksi, utilizing a plastic mold. The actual pressure used will depend upon the size of the billet being fabricated. With a 1 inch diameter billet, a pressure of 40 to 60 ksi can be employed while with 1.5 and 3 inch diameter billets the pressure ranges from 50 to 60 ksi. The sintering step is conducted under a vacuum at a temperature in the range of l,970 to 2,03 5 F for a period of about 2 to 4 hours. A vacuum of about 10 torr has been found to be satisfactory. It is critical that sinterin g of the green compact be conducted in the aforementioned temperature range. Thus, billets sintered at 1,900 F do not reach desired densities while those sintered at 2,100 F are essentially impermeable (too high density). Furthermore, a sintering time appreciably longer than 4 hours, such as 8 hours, is unsatisfactory because the billet product is much less uniform. Also, in order to obtain a billet having a uniform permeability, it is important that the sintering temperature be closely controlled so that variations within the billet are within 15 F.

In the preferred double-pressed process, the initial cold isostactic pressing step is usually conducted at a pressure ranging from 50 to 60 ksi. Thereafter, the green compact is annealed so as to promote stress relief therein. This permits further deformation by the second pressing step, thereby resulting in higher densities prior to sintering. The annealing heat treatment is generally conducted at about 1,350 F for a period of about 1 hour in order to promote stress relief without allowing possible non-uniform recrystallization in the green billet. After the annealing step, the billet is again subjected to cold isostactic pressing at a pressure in the range of 50 to 60 ksi. The annealed green compact is then sintered under a vacuum at a temperature in the range of l,970 to 2,035 F for a period of 2 to 4 hours. The criticality of the sintering temperature and time has been discussed hereinabove.

In Table 1 below, optimum processing conditions are shown for fabricating 1.5 inch diameter billets with permeabilities of1X10* 5X10 IXIO", 5X10 and 1X10 in and 3 inch diameter billets with permeabilities of 5X10'", 1X10 and SXIO. Included in the table are pressing pressures, sintering temperatures, and sintering times with corresponding maximum tolerances. Conditions for producing billets with permeabilities other than those specified can be readily obtained by interpolation.

tion by frequency and weight is listed below in Table 11.

TABLE 11 Size Range (p) By Frequency (71) By Weight (7() Polyurethane molds having a 2 inch inside diameter, a depth of 11 inches, and 0.125 inch wall thickness were employed for the 1.5 inch diameter billets. For the 3 inch diameter billets, polyurethane molds with a 3.375 inch inside diameter, a depth of 12 inches, and a wall thickness of 0.125 inch were used. The molds were first filled with powder and then additional powder was added continuously during 4 minutes of vibration at 60 Hz with an 0.20 inch amplitude. The powder-filled molds were then sealed with liquid cement and water proof adhesive tape. Double-pressed billets were returned to the molds after annealing and suffi- TABLE I Pressing pressure (k.s.l.) Sinterlng temperature, F. 1

Permeability (in 1.5 diam. 3 diam. 1.5" diam. 3 diam.

1(:l;0.1)) 10' 58 (3:1) 1,975 (15) 5(:h0.5)X10 59 (5:1) 59 (i1) 2,000 (i=5) 2, 025 ($5) 1(:l:0.1)X10 1 60/55 (il/l) 1 50/60 (;l;1/1) 2,010 (16) 2,020 (:1: 5(:l:1.0)X10 1 50/58 (11/1) 1 50/58 (11/1) 2, 020 (5:6) 2, 030 (5:5) 1(:1:0.2)X10' 1 50/60 (:lzl/l) 2,035 (i5) 1 Doub1e-pressed billets-arm 1 sintering timeof 4 hours =1; 10 minutes at indicated temperatures.

A more complete understanding of the invention can be obtained by referring to the following illustrative example which is not intended, however, to be unduly limitative of the invention.

EXAMPLE Runs were conducted in which 1.5 inch and 3 inch' diameter billets were prepared in accordance with the ealed for 1 hour at 1,350 F. before second pressing.

cient powder was added so that after 1 minute of vibration the molds were filled before rescaling and repressmg.

The conditions under which the runs were conducted in fabricating the 1.5 inch billets as well as their permeabilities and other properties are summarized below in Table 111.

TABLE III Compressive Sintered Tensile yie d yield strength Ultimate strength Tensi e modulus Pressing sintering 3 density, Gross strength (k.s.i.) (k,S.l.) (ksi.) (k.s.1.X10 pressure temperature percent permeability Billet N0 (ksi.) F.) theoretical (in?) 70 F. 700 F. 70 F. 700 F. 70 F. 700 F. 70 F. 700 F.

50 000 83. 8 5. 00X10- 20. B 15. 7 21. 4 18. 1 22. 1 18. 5 26. 2 19. 1 59 2, 000 83. 8 5. 28X10- 19. 9 16. 2 20. 7 16. 2 22. 0 19. 4 23. 7 27. 1 1, 975 79. 5 1. 25X10 l6. 7 13. 5 19. 9 15. 2 17. 7 15.1 17. 9 11. 6 60 1, 975 81.3 0.92X10 2 NA 16.6 10.5 13.8 1 NA 18.3 1 NA 20.5 1 50/60 2. 000 87. 8 1.15X10- 23. 9 19. 7 25. 7 19. 2 27.7 25. 2 29. 9 24. 5 1 50/56 2, 010 88. 0 0. 93X10- 23. 7 3 NA 26. 2 17. 9 28. 6 3 NA 22. 5 3 NA 1 50/60 2, 020 89. 4 3. 09Xl0- 25. 1 21. 4 27. 4 20. 2 32. 1 27. 8 27. 4 21. 2 1 50/58 2,020 89. 6 5.14X10- 23. 4 20. 1 26. 7 19. 8 29. 2 25. 6 26.1 27.4 1 50/60 2, 030 90. 5 1. 77 10- 25. 1 20. 4 27. 2 19. 3 30. 6 27. 2 27. 8 27. B 1 50/60 2, 91. 2 0. 77X10- 25. 5 1 NA 26. 9 18. 7 32. 0 7 N 28. 4 2 NA 1 Diouble-pressed billets-annealed [or 1 hour at 1,350" F. before second press g.

1 Test data not available.

1 Sinterlng time of 4 hours=l:10 minutes under a vacuum 0! about 10- torr.

The conditions under which the runs were conducted in fabricating the 3 inch billets as well as their permeabilities and other properties are summarized below in Table IV.

TABLE IV Sintered Yield strength Ultimate strength l'ercent Pressing Slntering 2 density, elongation pressure temperature percent (lross peluie- Long Trans Long Trans Billet No (ksi.) F.) theoretical ability (111. (ksi.) (k.s.i.) (ksi.) (ksi.) Long Trans 59 2, 000 80. 5 .1. 30X10- 59 2,010 82. 6 7. 27X10- 59 2, 020 82. 0 6. 32X10- 1 50/58 2,010 86.7 1. 50X10' 1 50/58 2.025 88. 8 5. 78x10- F. belore second pressing. m 01 about 10 torr.

In the runs the billets were pressed in a 60 ksi capacity, 4 inch diameter by inch high chamber isostactic ramic furance tube. Three stainless steel sheathed chromel-alumel thermocouples were positioned in a slot cut into the bottom of the graphite tube. The billets were in contact with the thermocouples which were disposed so that they monitored the temperature at the center and both ends of the billets. Temperature control was i5 F during a majority of the sintering time and uniformity within the billets was within fi" F.

The 3 inch diameter billets were sintered in a 6 inch diameter Marshall 3-zone horizontal tube furnace. The furnace was fitted with an inconel muffle containing a 16 inch long graphite liner in the center. Three stainless sheathed chromel-alumel thermocouples were positioned in a slot cut into the bottom of the graphite sleeve. The billets were placed in an inconel boat which was in contact with the thermocouples. The thermocouples-monitored the temperatures at the center and at both ends of the billets. Temperature control and uniformity in the billet during sintering was 1'5" F.

The viscous (a) and inertial (B) resistance coefficients and the permeability constant K l/a) were determined for 0.100 inch by 1 inch diameter disc specimens. After a minimum of l-.inch was cut from each end of the billets, the ,discs were machined from the ends. The following equation was used in making the permeability measurements:

AQEQZZkZRIAG r. 9 55 05.1- where: MP Upstream pressure squared minus downstream pressure squared. T Gas temperature. Z compressibility. L Specimen thickness. R Gas constant. G Mass flux. p. Viscosity. The gross permeability of each disc specimen was determined employing nitrogen gas flow measurements and a We inch diameter orifice. Measurements were made for 5 flow rate/pressure combinations. A computer program was used to fit the data to a leastsquares, second order curve representing the above equation.

The tensile tests were performed on a 10,000 pound capacity lnstron Model TT-C tensile machine, using a precision aligned, rigid load train system which enabled the tensile testing of brittle materials without exceeding 2 percent bending. The compression tests were also performed on the lnstron machine, employing a strain rate of 0.02 in/in/min.

From the data in the foregoing table, it is seen that the present invention makes it possible to produce beryllium billets with average gross permeabilities in the l l0- to 1X10 in range. However, the doublepressed process must be employed to obtain acceptably uniform billets with permeabilities below 5X10 in The single-pressed process can be used to produce billets having permeabilities of SXIO' in and above.

As will be evident to those skilled in the art, various modification of this invention can be made or followed in the light of the foregoing disclosure without departing from the spirit or scope of the disclosure.

I claim:

1. A process for fabricating a porous beryllium billet which comprises subjecting a mold filled with beryllium powder with particle sizes ranging between 74 and 10 microns to cold isostactic pressing at a pressure in the range of 50 to 60 ksi; annealing the resulting billet; subjecting the annealed billet to isostactic pressing at a pressure in the range of 50 to 60 ksi; and sintering the billet under a vacuum at a temperature in the range of l970 to 2035F for a period of 2 to 4 hours.

2. The process according to claim 1 in which the bi]- let is annealed by heating at a temperature of about 1,350 F for about 1 hour.

3. The process according to claim 2 in which the billet is sintered under a vacuum at a temperature in the range of 2010 to 2035F for a period of 2 to 4 hours.

4. A porous beryllium billet having a permeability in the range of l l0' to 1X10 in V V l= 

2. The process according to claim 1 in which the billet is annealed by heatiNg at a temperature of about 1,350* F for about 1 hour.
 3. The process according to claim 2 in which the billet is sintered under a vacuum at a temperature in the range of 2010 to 2035*F for a period of 2 to 4 hours.
 4. A porous beryllium billet having a permeability in the range of 1 X 10 10 to 1 X 10 12 in2. 